| Summary: | The gill, with a large surface area in intimate contact with the environment, is the primary organ of ion and acid-base regulation in fish. These same characteristics make the gill epithelium particularly susceptible to a number of environmental perturbations, including osmotic challenges and metabolic acidosis. For most freshwater fish, the active uptake of the strong ions, sodium and chloride, are intimately linked with the excretion of acid and base equivalents, respectively. However, fish from the genus Fundulus, are unique in their apparent lack of active chloride uptake at the gills. This unique feature makes limiting Cl- loss through paracellular pathways the only practical strategy for these species in tolerating acute exposure to hypoosmotic conditions especially when dietary chloride is limited. We find that Fundulus grandis can dynamically regulate their paracellular pathway in the gill epithelium at salinities approaching fresh water. We observe the significant up-regulation in the mRNA levels of several claudins in the gill and that some of these claudinsexhibit expressional discrepancies between mitochondrion-rich and pavement cells in fish gills, which shows correlations with changes in gill morphology and ion flux rates in fish. Collectively, our data suggest that claudins may play important roles in regulating ion flux across paracellular pathways of Fundulus during osmotic challenges.
The linkage between osmoregulation and acid-base tolerance has long been studied in teleost fish. Failure to identify an active Cl- uptake system in freshwater Fundulusindicates the uniqueness of these species in osmoregulation and acid-base tolerance. We find that Fundulus heteroclitus increases Na+ uptake within the first few hours of metabolic acidosis,We also find that metabolic acidosis induces the changes of mRNA levels of several gill claudin proteins, which may play roles in regulating the permeability of the paracellular pathway to strong ions. Our data show Fundulus heteroclitus is capable to cope with great metabolic acidosis within their bodies, which may contribute to their adaptation to internal and external perturbations.
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